This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The project involves an unusual protein disulfide isomerase, GILT, that works at acidic conditions in the lysosome. It is involved in mhc class II maturation and also plays a key role in the hi-jacking of cells by pathogens. We have selenomethionine substituted crystals of two differenet mutants with which we would like to use for phasing. These crystals can diffract to better than 3A on our home source and we expect better resolution at a synchrotron source. We have several pre-screened crystals and ~20 that we would also like to screen/collect on.The structural studies on GILT will help us to elucidate the enzymatic mechanism of the protein, and how it works at the acidic pH of the lysosome. Furthermore, this work will lay the basis to understand its involvement in pathogenic processes on a molecular level.